The present invention relates to a misfire sensing apparatus for an internal combustion engine, and more particularly, it relates to such a misfire sensing apparatus which is capable of sensing misfiring in any of cylinders of the engine as well as determining which cylinder is misfiring with improved accuracy.
Japanese Patent Laid-Open No. 62-118031 discloses a misfire sensing apparatus for an internal combustion engine in which the period of successive pulses per unit crank angle is measured, and a peak value of variations in the rotational speed of the engine is determined based on the pulse width of each pulse so that the time from a reference crank angle to the peak value is compared with a reference time which the peak value takes from the reference crank angle in the normal operation of the engine without misfiring, and an occurrence of misfiring as well as which cylinder such misfiring has taken place are determined based on the result of this comparison.
With such misfire and misfiring-cylinder determinations carried out by the conventional misfire sensing apparatus, however, variations in the rotational speed of the engine used for these determinations are subject to a significant change due to various factors other than misfiring such as, for example, influences from an engine drive system at the time when the vehicle is travelling on a bad road or the like. For this reason, too high sensitivity in misfire determination leads to an error or inaccurate result. On the other hand, if the sensitivity is low, it will become impossible to perform misfire determination with an acceptable level of accuracy at all times. That is, because of certain operating conditions of the engine, no misfiring can be determined in spite of the fact that a cylinder is actually misfiring.